1. Field of the Invention
The present invention relates to a magnetic position detection apparatus that magnetically detects a movement of a magnetic mobile object.
2. Background Art
There is a detection apparatus formed of a magneto-electric converting element and a magnet to magnetically detect a movement of a magnetic mobile object. The magneto-electric converting element referred to herein means an element whose electric resistance value varies with a magnetic field applied thereto, such as an MR (Magneto-Resistance) element. Because a magnetic field to be applied to the magneto-electric converting element by the magnet varies in association with a movement of the magnetic mobile object opposed to the magnet, the movement of the magnetic mobile object can be detected as a variance of the electric resistance value.
For example, a magnetic position detection apparatus disclosed in Japanese Patent No. 3682052 (Patent Document 1) is, as is shown in FIG. 19, provided to be spaced apart from a magnetic mobile object 500, which has a radially-protruding tooth portion on a circumference thereof and rotates in a circumferential direction, above a plane surface of the magnetic mobile object 500. The magnetic position detection apparatus includes a processing circuit portion 502 having a bridge circuit formed of first and second magneto-electric converting elements 501a and 501b, a magnet 503 that applies a magnetic field to the first and second magneto-electric converting elements 501a and 501b in a direction of a rotational axis line 504 of the magnetic mobile object 500, and a flux guide 505. When viewed along the direction of the rotational axis line 504, the second magneto-electric converting element 501b is disposed substantially on a center line in a circumferential width dimension of the magnet 503 while the first magneto-electric converting element 501a is displaced toward the magnetic mobile object 500 with respect to the second magneto-electric converting element 501b. Owing to this configuration, a differential output is obtained from outputs of the first and second magneto-electric converting elements 501a and 501b. 
Further, the flux guide 505 made of a magnetic material is provided between the processing circuit portion 502 and the magnet 503 to prevent dispersion of a magnetic flux. The flux guide 505 has a pair of protrusions mutually opposing with a spacing in the circumferential direction of the magnetic mobile object 500. The second magneto-electric converting element 501b is disposed substantially on a center line between a pair of the protrusions. The first magneto-electric converting element 501a is disposed on the side of one of the protrusions.
Patent Document 1: Japanese Patent No. 3682052
According to the magnetic position detection apparatus in the related art disclosed in Patent Document 1, the bridge circuit is formed of the first and second magneto-electric converting elements 501a and 501b and, as is shown in FIGS. 20A and 20B, the first and second magneto-electric converting elements 501a and 502a are disposed with a spacing N. Accordingly, there is a difference between times at which the first and second magneto-electric converting elements 501a and 501b are opposed to a tooth portion or a slot portion of the magnetic mobile object 500 in association with a movement of the magnetic mobile object 500. This time difference gives rise to a phase difference in variance of the electric resistance values of the respective magneto-electric converting elements 501a and 501b. Hence, a differential output of the bridge circuit varies more abruptly than in a case where the bridge circuit is formed of a single magneto-electric converting element (the other one is a resistor made of non-magnetic metal). Spatial resolution can be thus enhanced.
Incidentally, in a case where the bridge circuit is formed of a plurality of magneto-electric converting elements, when the electric resistance values of the respective magneto-electric converting elements are not equal, that is, in a case where detection sensitivities to a magnetic field of the respective magneto-electric converting elements and a magnetic field to be applied thereto are not the same, such differences appear as a differential output of the bridge circuit.
For example, in a case where magneto-electric converting elements have magnetic anisotropy, the sensitivity varies with a change of an angle at which the magnetic field is applied to the magneto-electric converting elements. Even when the magnetic position detection apparatus is present solely and not opposed to a magnetic mobile object, sensitivities of the respective magneto-electric converting elements differ due to a difference in angle at which the magnetic field is applied to the respective magneto-electric converting elements by the magnet. When magnetic anisotropy varies depending on performance of the magneto-electric converting elements, such a variance in magnetic anisotropy is reflected on a differential output of the bridge circuit formed of a plurality of the magneto-electric converting elements, thereby causing a variance of the differential output.
In a case where a magnetic position detection apparatus having a bridge circuit formed of a plurality of magneto-electric converting elements as in Patent Document 1 above is manufactured, a magnetic characteristic (sensitivity) of the magneto-electric converting elements and a combining position of the magneto-electric converting elements and the magnet (a magnetic field applied to the respective magneto-electric converting elements) vary from apparatus to apparatus in manufacturing. Accordingly, a variance of the sensitivity together with a variance of the magnetic field increases a variance of the differential output of the bridge circuit in comparison with cases where the bridge circuit is formed of a resistor made of non-magnetic metal or where the bridge circuit is formed of a single magneto-electric converting element and a resistor made of non-magnetic metal. The manufacturing variances may be controlled strictly or adjustments may be performed during the fabrication sequence as countermeasures. However, either countermeasure makes the manufacturing difficult and further increases the manufacturing costs.
Also, in a case where sensitivities differ among a plurality of magneto-electric converting elements forming the bridge circuit as described above, a difference in sensitivity also appears for temperatures. Because a temperature characteristic of a differential output of the bridge circuit is large, the temperature characteristic varies considerably.